1. Field of the Invention
This invention relates to an electrode to be used in a battery and to a battery using the same. In particular, it relates to a lithium ion secondary battery with improved safety.
2. Discussion of the Background
With the recent improvement of performance of electronic equipment, there has been a demand for improvement on batteries for use as a power source of the electronic equipment, especially rechargeable secondary batteries. Lithium ion secondary batteries have been attracting attention for their light weight, portability, and high capacity. Lithium ion secondary batteries, while advantageous for their high energy density, require sufficient safety measures because they use metallic lithium and a nonaqueous electrolytic solution.
A safety valve for escape of increased inner pressure and a PTC (positive temperature coefficient) element which increases resistance according to heat generation caused by an external short-circuit to cut off an electric current have been proposed to date as safety measures.
For example, JP-A-4-328278 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) proposes a cylindrical battery having a safety valve and a PTC element in the positive electrode cap. However, it is a generally followed practice that a safety valve is designed not to work so easily because, on the safety valve""s working, moisture in the air tends to enter the battery and react with lithium present in the negative electrode.
A PTC element, on the other hand, cuts off the circuit in case of an external short-circuit and causes no adverse influence. It could be the first safety component to work in case of abnormality by designing to work when the inner temperature reaches, for example, 120xc2x0 C. due to a short-circuit.
When a short-circuit occurs in the inside of a battery, the cutoff of the external circuit by a PTC element does not mean a cutoff of a short-circuit inside the battery. If a short-circuit occurs in the battery, and the inner temperature rises, the separator made of polyethylene, polypropylene, etc. which is interposed between a positive electrode and a negative electrode is expected to melt by the heat. The molten separator is expected to exude or enclose the nonaqueous electrolytic solution that has been held in the separator so that the ion conducting properties of the separator may be reduced to weaken the short-circuit current. However, a separator away from the heat generating part does not always melt.
In an attempt to solve the above problem, JP-A-7-161389 proposes using an active material having PTC characteristics in itself in the positive electrode. However, since the resistance of a positive electrode active material having PTC characteristics is about 10xe2x88x925 S/cm at a working temperature (room temperature), the battery will not function unless such a positive electrode active material is used in combination with an electrical conduction aid as demonstrated in Examples of the publication. With a conduction aid having no PTC behavior being added, even though a positive electrode active material exhibits PTC characteristics, a short-circuit current is to flow via the conduction aid.
An object of the present invention is to settle the above-mentioned problems and to provide a highly safe lithium ion secondary battery which controls heat generation in case of an external and/or internal short-circuit.
A first aspect of the present invention is an electrode which comprises:
an electrode active material layer containing an active material;
and an electronic conducting current collector on which the electrode active material layer are formed wherein said electrode active material layer has the property of increasing its resistance with a rise in temperature (PTC).
A second aspect of the present invention according to the first aspect is an electrode wherein said active material has the property of increasing its resistance with a rise in temperature (PTC).
A third aspect of the present invention according to the first aspect is an electrode wherein said electrode active material layer comprises an active material particle and an electronic conducting material, and the electronic conducting material has the property of increasing its resistance with a rise in temperature (PTC).
A fourth aspect of the present invention according to the first aspect is an electrode wherein said electrode active material layer is composed of an active region having an electrode activity and a non-active region having no electrode activity by which said active region is isolated into a plurality of parts, and said non active region has the property of increasing resistance with a rise in temperature.
A fifth aspect of the present invention according to the first aspect is an electrode wherein said electrode active material layer is composed of an active region having an electrode activity and a non-active region having no electrode activity by which said active region is isolated into a plurality of parts, and said active region has the property of increasing resistance with a rise in temperature.
A sixth aspect of the present invention according to the first aspect is an electrode wherein said active material is made up of secondary particles comprising a plurality of active material particles having adhered on the surface thereof electronic conducting particles having the property of increasing resistance with a rise in temperature.
A seventh aspect of the present invention according to the first aspect is an electrode wherein said electronic conducting current collector has the property of increasing its resistance with a rise in temperature (PTC).
An eighth aspect of the present invention according to the seventh aspect is an electrode, wherein said electronic conducting current collector is composed of conductive plate to which an electronic conducting material having the property of increasing resistance with a rise in temperature is bonded.
A ninth aspect of the present invention according to the sixth aspect is an electrode, wherein said electronic conducting material is a polymer having a softening point not higher than 150xc2x0 C.
A tenth aspect of the present invention is a battery which comprises: a positive electrode; a negative electrode; and an electrolyte provided between said positive electrode and said negative electrode, wherein at least one of the positive electrode and negative electrode comprises an electrode active material layer containing an active material and an electronic conducting current collector on which the electrode active material layer are formed, and said electrode active material layer has the property of increasing its resistance with a rise in temperature (PTC).
An eleventh aspect of the present invention according to the tenth aspect is a battery, wherein said electrode active material layer is composed of an active region having an electrode activity and a non-active region having no electrode activity by which said active region is isolated into a plurality of parts, and said non active region has the property of increasing resistance with a rise in temperature.
A twelfth aspect of the present invention according to the tenth aspect is a battery, wherein said electrode active material layer is composed of an active region having an electrode activity and a non-active region having no electrode activity by which said active region is isolated into a plurality of parts, and said active region has the property of increasing resistance with a rise in temperature.
A thirteenth aspect of the present invention according to the tenth aspect is a battery, wherein said active material is made up of secondary particles comprising a plurality of active material particles having adhered on the surface thereof electronic conducting particles having the property of increasing resistance with a rise in temperature.
A fourteenth aspect of the present invention according to the tenth aspect is a battery, wherein said electronic conducting current collector has the property of increasing its resistance with a rise in temperature (PTC).
A fifteenth aspect of the present invention according to the fourteenth aspect is a battery, wherein said electronic conducting current collector is composed of metal to which an electronic conducting material having the property of increasing resistance with a rise in temperature is bonded.
A sixteenth aspect of the present invention according to the thirteenth aspect is a battery, wherein said electronic conducting material is a polymer having a softening point not higher than 150xc2x0 C.
A seventeenth aspect of the present invention is a battery which comprises:
an electrode active material layer containing an active material;
and an electronic conducting current collector on which the electrode active material layer are formed,
wherein said electronic conducting current collector has the property of increasing its resistance with a rise in temperature (PTC) electrode active material layer and
said electrode active material layer is made of an plurality of parts electrical isolated each other.
A eighteenth aspect of the present invention according to the seventeenth aspect is a battery, wherein said electronic conducting current collector is made of a conductive plate to which an electronic conducting material has the property of increasing its resistance with a rise in temperature (PTC) is bonded.
A ninteenth aspect of the present invention is a battery which comprises: a positive electrode; a negative electrode; and an electrolysis held between said positive electrode and said negative electrode,
wherein at least one of the positive electrode and negative electrode comprises an electrode active material layer made of an plurality of parts electrical isolated each other and an electronic conducting current collector on which the electrode active material layer are formed,
and said electronic conducting current collector has the property of increasing its resistance with a rise in temperature (PTC).